Raise custom Exception with arguments

Question

I\'m defining a custom Exception on a model in rails as kind of a wrapper Exception: (begin[code]rescue[raise custom exception]end)

When I raise the Exc

Answer 1

TL;DR 7 years after this question, I believe the correct answer is:

class CustomException < StandardError
  attr_reader :extra
  def initialize(message=nil, extra: nil)
    super(message)
    @extra = extra
  end
end
# => nil 
raise CustomException.new('some message', extra: "blupp")

WARNING: you will get identical results with:

raise CustomException.new(extra: 'blupp'), 'some message'

but that is because Exception#exception(string) does a #rb_obj_clone on self, and then calls exc_initialize (which does NOT call CustomException#initialize. From error.c:

static VALUE
exc_exception(int argc, VALUE *argv, VALUE self)
{
    VALUE exc;

    if (argc == 0) return self;
    if (argc == 1 && self == argv[0]) return self;
    exc = rb_obj_clone(self);
    exc_initialize(argc, argv, exc);

    return exc;
}

In the latter example of #raise up above, a CustomException will be raised with message set to "a message" and extra set to "blupp" (because it is a clone) but TWO CustomException objects are actually created: the first by CustomException.new, and the second by #raise calling #exception on the first instance of CustomException which creates a second cloned CustomException.

My extended dance remix version of why is at: https://stackoverflow.com/a/56371923/5299483